1. Field of the Invention
The present invention relates to a control apparatus for an electric motor driven by a load current modulated to a pulse form by a switching device, in which an average value of the load current is made to follow a command value.
2. Description of the Related Art
In order to control the operation of a DC electric motor by modulating a motor current flowing in the motor, that is, a load current to a pulse form, and by varying a pulse width so that an average value of the load current follows a command value, the detection of the average value of the load current is required. However, in such a control, since the load current pulsates, it is a problem how to detect the average value of the load current in a control period. In one of prior art methods for detecting the average value of the load current, a value of the load current when a switching device applying a voltage to the motor is changed to an "on" or "off" state is detected, and using this value as an initial value, the average value of the load current is estimated on the basis of a variation characteristic of the load current determined by a power supply voltage, motor constants, and a switching period (e.g., Japanese Patent Laid-Open Publication No. 58-58893 (1983)).
However, the aforementioned method poses a problem in that since a range of the estimated values is generally large, a detection error of the average value becomes large. In particular, when the load is not applied to the electric motor, the error becomes large. Furthermore, there is another problem in that in order to perform the estimation calculation, a burden on a CPU is heavy, and at the same time, when the calculation time is long, the motor control in a short period becomes impossible.